Embodiments of the invention relate to Near-Field or NFC communication devices, and more particularly to the antenna circuits used in such devices.
FIGS. 1A to 1C represent various known electrical structures of antenna circuits. These antenna circuits are usable in active NFC devices able to create a magnetic field and to detect its modulation (for example an RFID tag reader), and in passive NFC devices (for example an RFID tag) powered by a magnetic field and configured to modulate this field in order to transmit data.
FIG. 1A illustrates the structure most generally used in NFC devices. It includes a single winding L1 connected in parallel with a tuning capacitor C. The antenna circuit is tuned, for example, to 13.56 MHz within the framework of the NFC standards in force (e.g. ISO 14443, ISO 15693). The terminals of the winding form the access terminals A1, A2 of the antenna circuit.
The turns of the antenna windings of NFC devices are generally produced in the form of metallic tracks etched spirally in a single layer of a flexible printed circuit. The windings are plane and coplanar. This makes it possible to minimize the cost of manufacture, but entails certain difficulties due to the fact that the turns are all of different dimensions and therefore do not have similar characteristics.
FIG. 1B illustrates an optimized structure, described for example in Japanese patent application JP 2000-278172. The antenna winding includes a midpoint forming one of the access terminals (A1) of the antenna circuit. It is considered that the antenna winding is formed of two windings connected in series L1 and L2, the access terminals A1 and A2 being taken at the terminals of the winding L1, which will be referred to as the “active winding”. The winding L2 will be referred to as the auxiliary winding. These terminals A1, A2 are generally connected to an NFC circuit (not shown), which may be a passive circuit (receiving the magnetic field emitted by another NFC device) or an active circuit (emitting the magnetic field).
As indicated in the aforementioned document, this structure makes it possible to increase the number of turns of the antenna winding and consequently to increase the energy received or emitted by means of the winding by the NFC circuit connected to the terminals A1, A2, while decreasing the number of turns seen by the NFC circuit, so that the impedance of the coil seen from the terminals A1, A2 is closer to the output impedance of the NFC circuit, thereby improving the quality factor Q of the antenna circuit.
FIG. 1C illustrates an alternative structure to that of FIG. 1B. The antenna winding includes two midpoints forming the access terminals A1 and A2, also connected to an NFC circuit. It is considered that the antenna winding is formed of three windings connected in series L1, L2 and L3, the access terminals A1 and A2 being taken at the terminals of the winding L1. The winding L1 is the active winding, and the windings L2 and L3 are auxiliary.
FIG. 2A schematically represents a conventional spatial configuration of the antenna circuit of FIG. 1B. The whole antenna winding is coiled in a spiral, in a single plane. To simplify the drawing, it is assumed that the winding L1 includes one turn and the winding L2 two turns. The antenna winding thus includes three turns. The inner turn, shown in solid line, is that of winding L1 and the two outer turns, shown in dotted lines, are those of winding L2.
FIG. 2B schematically represents a conventional spatial configuration of the antenna circuit of FIG. 1C. As previously, the whole antenna winding is coiled in a spiral, in a single plane. To simplify the drawing, it is assumed that each winding L1 to L3 includes a single turn. The antenna winding thus includes three turns. The inner turn, shown in dotted line, is that of winding L2 and the outer turn, shown in chain dotted line, is that of winding L3. The middle turn, shown in solid line, is that of the active winding L1.
It has been demonstrated that the circuit of FIG. 1B achieved according to the spiral spatial configuration of FIG. 2A offers, in terms of communication distance, lower performance than that of the circuit of FIG. 1C achieved with the same dimensions according to the same spiral spatial configuration (FIG. 2B).